The present disclosure relates to semiconductor memory devices and, more particularly, to a semiconductor memory device controlling an output voltage level of a high voltage generator in accordance with variations in temperature.
Semiconductor memory devices are storage units capable of containing data, and from which the data are read out as needed. Semiconductor memory devices may be briefly classified into random access memories (RAMs) and read-only memories (ROMs). RAMs are kinds of volatile memory devices that lose their data in the absence of a power supply. ROMs are nonvolatile memories that retain their data even without power supply. RAMs include dynamic RAM, static RAM, and so on. ROMs include programmable ROM (PROM), erasable and programmable ROM (EPROM), electrically EPROM (EEPROM), flash memory, and so on. Flash memories are divided into NAND and NOR types.
Further, a semiconductor memory device usually employs an internal high voltage generator for producing a high voltage, for example, 9V, from a lower power source voltage, for example, 1.2V. Such a high voltage provided from the high voltage generator is usually used to carry out a programming or erasing operation. For instance, a NAND flash memory requires a voltage of about 18V for programming and a voltage of about 20V for erasing. Furthermore, a NOR flash memory needs a voltage of about 5V (in a stack-gate memory cell) or about 9V (in a split-gate memory cell), for programming.
In a general flash memory device, it is required to apply a correct high voltage level to the memory cells during a programming or erasing operation. Variations of processing conditions in fabricating semiconductor chips, however, cause high voltage levels to be irregular according to their positions in manufacturing lots or even in the same wafer. For the purpose of solving such a problem, such high voltages are trimmed to desired levels, respective to the chips, during chip test operations. Nevertheless, an output level of the high voltage generator will fluctuate in accordance with temperature variations, as well as variations of processing conditions. If a high voltage output becomes irregular caused by variations of temperature, it results in deterioration of program and erasure characteristics.